ABSTRACT- The toxicity of contaminated Lake Superior sediments from Ashland, WI, was evaluated as part of an ecological risk assessment. Toxicity tests with field-collected sediment samples were conducted in 1998 and 2001 using solid-phase sediment and sediment elutriates, and infaunal, epibenthic, and planktonic invertebrates, as well as a fish species as test organisms. Effect concentrations for total PAHs from tests without UV exposure [i.e., 10-d solid-phase tests of undiluted sediment with Hyalella azteca, Chironomus tentans and Lumbriculus variegatus, a 10-d solid-phase test of diluted sediment (mixed with uncontaminated sediment in a dilution series) with C. tentans, and a 28-d diluted sediment test with H. azteca], agreed well with effect concentrations in various sediment quality guidelines. Since the sediment PAH mixture contained several UV-excitable compounds, studies of possible photoactivated toxicity were conducted. Tests with L. variegatus, Daphnia magna and Pimephales promelas, in which initial sediment or elutriate exposures were followed by exposure to UV light ( 520 and 100 microwatts/cm2 of UVA and UVB, respectively), demonstrated increased toxicity with UV exposure. A 28-d H. azteca test with simultaneous sediment and UV exposures, with UV intensity levels (i.e., 50-80 and 2-6 microwatts/cm2 for UVA and UVB, respectively) simulating those measured at the study site, showed greater toxicity with UV exposure than without UV. This combined sediment and UV exposure provided a useful line of evidence for establishing the effects range for total PAHs in the sediment.